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Geological Magazine

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Figure 9. Fissure 2 showing the morphological evolution of fissure 2 as it was quarried from NW to SE. Overall there is a change from large clast breccias and conglomerates at 0 m to smaller clast reddish conglomerates in the middle section and to grey-black conglomerates and siltstones in the last sections at 23–26 m. The main fissure is accompanied by a brown sand and mud-filled satellite fissure development which can be followed from 0 m to the section at 11 m. The first (discovery) exposure of the fissure at 0 m is a complex fill with thick laminated red sandy marls succeeded by grey laminated Westbury-type sediments that yielded palynomorph assemblage FP2.1. This sequence continued upwards with fining-up breccia and conglomeratic sediments that included Westbury-type grey sandy mudstones; this is the rock that yielded the original find of Thecodontosaurus at Tytherington. At 5 m, the exposure shows a similar fill (where exposed) with the upper part of the fill separated into two. This separation is lost in the next exposure (8 m), which is a repeating sequence of conglomerates and finer reddish sediments. Some of the conglomerates, which are identical to ‘dolomitic conglomerate’, have large rounded clasts of sandstone and limestone including dolomitized limestone. At 11 m the lower part of the fissure has become simply a vertical joint but the upper part is much wider and a continuation of the section at 8 m. The upper infilling is also similar to that at 8 m with alternating beds of large and small clast conglomerates. The large clast conglomerates then disappear by 14 m where the fill is solely composed of red small-clast conglomerates. At 14 m and 17 m there is the complex development of a short phreatic tube. The section at 17 m has been interpreted by some (e.g. Simms, 1990) as a phreatic tube with a vadose notch cut into the base. It is more likely to have been formed by tectonic and solutional activity along a vertical joint entirely below the water table. The very localized circular structure would have been in direct receipt of surface waters that swirled around at the freshwater lens/saline water boundary to create the feature (see Fig. 16 and Whiteside & Robinson, 1983). A comparison with the previous cross-section at 14 m demonstrates that the right hand side of the ‘tube’ at 17 m was produced later than the left hand side and is strong evidence for void formation during the Penarth Group. The void was then filled by ‘glauconitic clay’-rich conglomerates on the right hand side of the ‘tube’ at 17 m. The fissure rapidly closes to an enlarged joint by 20 m and remains so until the final section at 26 m. In parallel with this closure, the satellite fissure has opened. The satellite fissure seen at 23 m is likely to be an extension of fissure 1 and implies that there was an inter-connectivity of the two systems. The locations of the three fissure 2 samples are shown. FP2.2 can only be positioned approximately between 17 m to 18 m. These series of sections were compiled from in situ collection and measurements, photos and field notes made between successive quarry blasts. The top of the section is hung on the top of the third quarried level which is shown as the ‘horizontal’ lines on the second exposure at 5 m (29/11/75). The base of the fissure remains unknown, but as these fissures were not located during the quarrying of the fourth level it may lie in the upper part of this deepest level (at approximately 60 m OD; about 44 m below the Carboniferous Limestone surface).

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